1. Field of the Invention
The present invention relates to a scintillator panel, a radiation detection device, and a radiation detection system, and more particularly a scintillator panel, a radiation detection device, and a radiation detection system for, for example, a medical X-ray imaging apparatus, an X-ray imaging apparatus for an industrial nondestructive test, or the like.
Note that, it is assumed in this specification that various electromagnetic waves such as an X-ray, an α-ray, a β-ray, and a γ-ray are included in a category of radiation and the description will be made on the basis thereof.
2. Related Background Art
Recently, digitization in the medical machine market is accelerated. Also, with respect to an X-ray image pickup system, a paradigm shift from a conventional film screen system to an X-ray digital radiography system is progressed.
Of X-ray detection devices for an X-ray image pickup having an X-ray digital radiography system, there is a device in which a sensor panel and a scintillator are adhered to each other through an adhesion layer made of a transparent adhesive. Note that the sensor panel includes a photoelectric conversion element portion having a photosensor using amorphous silicon and the like and a TFT. The scintillator includes a phosphor layer made of a fluorescent substance and a reflective film such as a metallic thin film for reflecting visible light emitted from the phosphor layer to the sensor panel side.
With respect to such an X-ray detection device, various elements can be combined according to a purpose without limiting an element structure of the sensor panel and a fluorescent substance material of the scintillator.
Next, an operation of the X-ray detection device will be described. First, when an X-ray enters the main body of the device, it is transmitted through the reflective layer and absorbed in the phosphor layer. After that, the phosphor layer emits visible light having an intensity corresponding to the absorbed X-ray. The visible light is converted into an electrical signal by the photosensor in the photoelectronic conversion element portion and outputted to the outside in accordance with switching of an on/off of the TFT. Thus, information of the input X-ray is converted into a two-dimensional digital image.
Here, various base members composing the scintillator are considered. However, it is preferable that amorphous carbon or the like is used because of the following reasons.    (1) Since the absorption of an X-ray is small as compared with glass and aluminum, a larger amount of X-rays can be emitted to the phosphor layer side. For example, in the case where respective materials are set to be practical thicknesses (OA-10 glass plate produced by Nippon Electric Glass Co., Ltd.: 0.7 mm, Al plate: 0.5 mm, and amorphous carbon plate: 1 mm), when photon energy is 60 keV or higher in any materials, transmittance of 90% or higher can be kept. However, transmittance is greatly reduced in the case of 60 keV or lower in the OA-10 glass plate and in the case of 35 keV or lower in the Al plate. On the other hand, although the amorphous carbon plate is thicker than other materials, transmittance of 95% or higher is kept in the case of 20 keV or higher. Thus, a nearly flat transmittance characteristic can be indicated within an energy region of an X-ray used in a medical field.    (2) Amorphous carbon has a superior medicine resistance. There is no case where amorphous carbon is eroded by strong acid such as hydrofluoric acid and a solvent.    (3) Amorphous carbon has a superior heat resistance. The amorphous carbon has a higher heat resistance than glass and aluminum.    (4) Amorphous carbon has a good conductive property. Since the amorphous carbon has a conductivity σ of 2.4×10−2 Ω−1cm−1, it also serves as an electromagnetic shield and for preventing electrostatic discharge in manufacturing.    (5) Since the thermal expansion coefficient is close to that of glass, when amorphous carbon is bonded to a base member made of glass etc., a possibility of peeling and the like by a difference of an expansion coefficient after bonding is low. Although thermal expansion coefficient of generally used panel glass is 4.6×10−6, that of amorphous carbon is close to this value and 2.0×10−6.
Also, the reason why the reflective layer is used is as follows. That is, since the reflectance of amorphous carbon or the like to an air layer is about 20% and low, the reflective layer made from a metallic thin film is provided to improve light utilization efficiency. Various materials are considered for a material for the reflective layer. However, it is preferable that a metallic film made of aluminum etc. is used as a material for the reflective layer because of the following reasons.    (1) High reflectance is indicated approximately through the entire region of visible light. Note that detailed information is described in Journal of the Optical Society of America, Vol. 45, No. 11, p 945, 1955.    (2) It is a low cost.    (3) A thin film formed by evaporation is easy to obtain a mirror surface and the occurrence of disturbance of resolving power due to diffuse reflection is less.
Also, a scintillator including these materials is concretely manufactured by the following method. First, a base member made of amorphous carbon or the like, whose surface is polished to be a mirror surface is washed and then an aluminum thin film is formed thereon by sputtering or the like. When the aluminum thin film is too thick, diffuse reflection is caused by uneven portions in the surface. On the other hand, when it is too thin, light is transmitted. Thus, the thickness is generally set to be 100 nm to 500 nm.
Next, a column-shaped phosphor layer is formed on the aluminum thin film by evaporation. A process temperature at this time exceeds 200° C. in many cases. After that, a protective layer is formed around the phosphor layer to complete a scintillator.
However, the following was cleared from our studies. That is, in the above-mentioned method, when alkali halide phosphor, for example, CsI is formed on a reflective layer which is formed in a conductive base member made of amorphous carbon or the like, corrosion of the reflective layer is started within several days. As this reason, it is considered that aluminum as a material for the reflective layer is corroded by halogen in CsI, that is, iodine.
As one method of preventing this corrosion, it is considered that a protective layer is provided in a front surface side of the reflective layer. However, it is found that corrosion caused within a short time cannot be suppressed. Also, with respect to such a problem, it is found that the occurrence of corrosion is greatly suppressed in the case where glass is used as a material for the base member and aluminum is used as a material for the reflective layer.
Thus, it is considered that electrochemical corrosion caused in the case where a conductive material (such as a material including a carbon component, for example, amorphous carbon or a material including a silicon component) and a different kind of conductive material for a reflective film of metal such as aluminum are laminated is greatly related to another reason why the reflective layer is corroded.
Here, according to Japanese Patent Application Laid-open No. 53-122356, it is described that a phosphor made of cesium iodide is provided on the entire surface of a substrate through an aluminum evaporation film. However, from the same reason as above, electrochemical corrosion cannot be prevented by the technique described in this document.
Also, according to Japanese Patent Application Laid-open No. 10-160898, the structure using an insulator such as PET or glass as a base member is disclosed. However, since the base member itself is an insulator, electrochemical corrosion is not almost caused between the base member and a reflective layer.
As described above, the electrochemical corrosion of the reflective layer in the scintillator panel, that is, corrosions of the reflective layer and the phosphor layer due to reaction between the base member and the reflective layer becomes a problem to be solved in order to realize a scintillator panel having high reliability for a long period.
In addition, when a reflective film made of Al or the like is directly formed on a base member made of amorphous carbon or the like by evaporation, since the adhesion to the surface of the amorphous carbon is not preferable, there is a problem in that peeling is caused in an interface between the base member and the reflective layer. There may be the case where this also becomes a problem when realizing a scintillator panel having high reliability.